Current collector foils often consist of an aluminium foil and additionally have a coating, which generally consists of a metal oxide, for example a lithium cobalt (III) oxide. An aluminium foil will be understood hereinafter to mean a foil consisting of aluminium or an aluminium alloy. During their production, aluminium foils are generally wetted with a rolling oil during the rolling process, for example so as to minimise friction effects, roller wear, or sticking of the rolled product during the rolling operation. However, the rolling oil must generally be removed again before further use of the aluminium foil. In particular with the use of aluminium foils for lithium-ion accumulators, for example as a current collector foil, the quality of the aluminium foil affects the quality of the produced lithium-ion accumulators. It is known from the prior art to clean, in particular to degrease, aluminium foil coated with rolling oil by subjecting the aluminium foil in the form of foil windings, so-called “coils”, to a thermal treatment. However, the hard-rolled foil softens and recrystallizes according to temperature and treatment time. The aluminium foil loses a considerable amount of strength and is more difficult to process. In addition, the oxide layer thickens as a result of the heat treatment due to the influence of temperature and oxygen, which leads to an increased electrical resistance of the current collector foil. If the rolling oil is not removed completely, problems with regard to the adhesion of the metal oxide layer may also be encountered.
In order to solve the problem of softening, it is proposed in published patent application US 2009/0269609 to carry out a thermal treatment at temperatures between 80° C. and 160° C. However, it has been found that, depending on the temperature, time, winding structure of the coil and the surface roughness of the aluminium foil, thermal treatment cannot remove the rolling oil completely from a foil winding. Particularly at the low temperatures of 80° C. to 160° C., there is a risk that rolling oil residues remain on the aluminium foil, which is problematic for further processing, for example coating with metal oxide.
In addition, acid or alkali cleaning of aluminium strips, for example in a coil-to-coil method, is known. Aluminium strips are generally more than 100 μm thick, usually more than 200 μm thick. However, current collector foils are more than 2 to 8 times thinner than the aluminium strips cleaned previously by means of wet-chemical methods.
An aluminium current collector foil which is used for example in rechargeable batteries is known for example from JP 2008 258010 A. A degreasing treatment is carried in order to clean the aluminium foil after rolling from oil, wherein acids and bases can be used. These are used for dissolving the oil without causing damage to the material.
On this basis, the object of the invention is to provide a current collector foil, which has very good properties with regard to conductivity and tensile strength, and which can also be produced economically. In addition, the object of the invention is to propose a method for producing the current collector foil as well as advantageous uses of the current collector foil.